1. Field of the Invention
The invention relates generally to ultrasonic and radiographic non-invasive methods for examining tissue or other solids. In particular, the invention relates to the coordination or fusion of ultrasonic monograms with x-ray or other radiographic imaging techniques to aid in the detection, location and biopsy of micro-calcifications in a human breast.
2. Description of the Related Art
Various means of non-invasive imaging are useful in medicine and other fields for visually modeling the interior structure of a solid subject body. For example, a very common method of screening women for breast cancer is x-ray mammography. Ultrasonic imaging is another, less common technique for examining breast tissue.
X-ray mammography provides excellent detection of certain types of tissues, but nevertheless has shortcomings. This technique provides detailed image information about well differentiated materials within the body (such as bone or other calcified tissue), but it performs poorly at discriminating between soft tissues with subtle differences in density and structure. Some women have mammographically dense breasts, as compared to more fatty breasts; there is a substantially increased risk of missing breast cancers when diagnosing such women by x-ray. The use of x-rays for examination also necessarily results in the exposure of the patient to ionizing radiation, which has well know associated risks. The technique is also limited in that it projects three-dimensional structure onto a two-dimensional plane, and thus does not capture the elevation or depth (position in the direction of radiation propagation) of features of interest
A newer imaging technique, ultrasonic imaging, is widely used for diagnosis in numerous medical fields. When properly used and adjusted, an ultrasound imaging system can non-invasively provide a cross-sectional view of soft tissue being imaged, such as the tissue of a breast, heart, kidney, liver, lung, eye, abdomen, or pregnant uterus.
A typical ultrasound imaging device operates by directing short ultrasonic pulses, typically having a frequency in the range of 1-30 MHZ, into the tissue being examined. The device then detects responses such as echoes, harmonics, phase or frequency shifts, of the ultrasonic pulses caused by acoustic impedance discontinuities or reflecting surfaces within the tissue.
A typical scanhead for an ultrasonic imaging system has a linear array of ultrasonic transducers which transmit ultrasonic pulses and detect returned responses. The array of transducers provides simultaneous views of the tissue at positions roughly corresponding to the positions of the transducers. The delay time between transmitting a pulse and receiving a response is indicative of the depth of the discontinuity or surface which caused the response. The magnitude of the response is plotted against the position and depth (or time) information to produce a cross-sectional view of the tissue in a plane perpendicular to the face of the scanhead.
Sophisticated ultrasonic imaging systems are available which are capable of volume reconstruction by assembling information from multiple two-dimensional cross-sections to form a three dimensional representation of subject tissue. For example, one such system is described in U.S. Pat. No. 5,787,889 to Edwards et al. (1998). An enhanced ultrasound imaging system employing targeted ultrasound is described in U.S. Pat. No. 5,776,062 to Nields (1998) Such systems are potentially useful in the diagnosis of suspicious lesions in the breast. The system of Nields can also be used to guide the biopsy of a potential lesion or suspicious mass in a breast. Compared to x-ray techniques, such ultrasonic techniques are advantageous in that the patient is not exposed to radiation. Ultrasound is also superior for imaging many types of soft, low-density xe2x80x9chidden massesxe2x80x9d which are typically invisible or very obscure in x-ray imagery. On the other hand, the lower resolution of ultrasonic imaging (compared to x-ray) makes it difficult or impossible to identify fine features, such as hard micro-calcifications in breast tissue, which would be visible in an x-ray.
Imaging of small calcifications is particularly useful because such calcifications play an important role in the detection of breast cancer. They are typically categorized as either benign, probably benign, or suggestive of malignancy, based on a number of factors including size, shape, and distribution. Mammographically detected calcifications are frequently the only detectable sign of breast cancer, so their proper investigation is crucial. While some benign calcifications cannot be distinguished from those associated with malignancy, many can be so distinguished by their patterns and distribution. If more of these benign calcifications could be detected and characterized by careful analysis, the number of biopsies for benign conditions could be decreased. Therefore, any imaging technique which can enhance the analysis is extremely useful.
Although the smallest micro-calcifications are virtually impossible to detect by ultrasound, larger micro-calcifications, for example those of around 50 micron or greater size, do measurably affect ultrasound propagation (provided that short wavelengths are used). However, their images are not easily perceived in ultrasonographic imagery, because of a characteristic of ultrasonographic imagery called xe2x80x9cspecklexe2x80x9d or xe2x80x9cspeckle noisexe2x80x9d. Random or disorganized sound reflection and interference cause ultrasonographic images to display a speckled or grainy texture. A closely analogous phenomenon is commonly observed when coherent light is used to view an irregular surface: the smooth surface appears grainy or speckled.
The speckle phenomenon tends to obscure the reading of ultrasonographs to detect micro-calcifications. The target micro-calcifications commonly occur as discrete, small individual members of a larger xe2x80x9cconstellationxe2x80x9d or cluster (the shape of which depends on the type of calcification and its cause). By an unhappy coincidence, it so happens that the size of the individual micro-calcification members is often similar to the characteristic speckle size in many sonographs. In the case of multiple scattering sites per unit volume, the size of the characteristic speckle is a affected much more by the characteristics of the beamforming apparatus than the structure of the tissue being examined. Although a constellation may be present, its recognition is made difficult by the speckle noise in which the pattern is imbedded.
Another problem with breast imaging is the difficulty of combining multiple image modalities. Given that ultrasound and x-ray techniques have somewhat complementary imaging capabilities, it is often desirable to use both techniques to obtain the most imformation possible. Although a patient (or other subject body) can be subjected to multiple imaging techniques (for example x-ray and ultrasound), the images obtained are not easily registered or correlated with one another. Differences in scale, position, or in the orientation of the plane of projection (of a two-dimensional image) are almost inevitable.
U.S. Pat. No. 5,531,227 to Schneider (1996) discloses a method and apparatus for obtaining an image of an object obtained by one modality such that the image corresponds to a line of view established by another modality. However, the method disclosed requires one or more fiducial markers to inter-reference the images. The preferred method disclosed also involves mounting the patient""s head immovably to a holder such as a stereotactic frame, which is inconvenient for the patient and the technicians. The method identifies fiducial markers by digital segmentation, feature extraction, and classification steps, which would most suitably be performed with powerful digital hardware and custom software. The method disclosed will perform best with fiducial markers which are easily automatically recognized, as by some simple geometric property; it is described in connection with using circular eye orbits as fiducial markers. In a human breast, however, such natural geometric features may not be readily available.
Another method of correlating ultrasonic image data with radiographic image data is disclosed in U.S. Pat. No. 5,640,956 to Getzinger et al (1997). This method requires that an x-ray image be obtained while the tissue is in the same position as it was while the ultrasonic data was being gathered. It also requires the use of fiducial reference markers (preferably multiple x-ray opaque reference markers). The method and apparatus described by Shmulewitz in U.S. Pat. No. 5,664,573 similarly requires that the breast be maintained in the same position (relative to the apparatus) during both the mammogram exposure and the ultrasound imaging. Essentially, a mammographic system and an ultrasound imaging system are combined into a single, combined unit. Such a unit is bulky and necessarily expensive.
U.S. Pat. No. 5,662,109 to Hutson describes a different approach to correlating and fusing ultrasonographic and mammographic data from the same breast. In Huston""s invention, an enhancer receives, combines, and correlates the two sets of data by a matrix manipulation technique. The enhancer embeds the sets of data in matrices and uses singular value decomposition to compress the data into singular vectors and singular values. The compressed data can then be altered to enhance or suppress desired features before display. This approach is promising but is very computationally demanding.
The invention is an apparatus and method for quickly coordinating ultrasonographic information about the internal structure of a solid subject body with x-ray or other radiographic information taken from the same subject body. The apparatus and method are especially suited for detecting and enhancing visualization of micro-calcifications in human breast tissue.
Given a radiographic transmission image of a subject body, and given further a set of volumetric, three-dimensional image data of the same subject body, the invention relates a region in the original radiographic image to a region within the three-dimensional image data by using a two-dimensional image cross-correlation, preferably performed by an optical correlator. In the preferred embodiment, the invention also uses a two-dimensional cross-correlation to find the elevation of a feature of interest in the three-dimensional data set.
In one embodiment, after coarse alignment of the images the invention locates, enhances, and displays small calcifications in a specified sub-region of the ultrasonographic image. Ultrasonographic imaging of small calcifications or similar hard bodies is enhanced by correlating an ultrasonographic data set with a radiographic image (preferably pre-processed and enhanced) of the same region of interest. A xe2x80x9cconstellationxe2x80x9d or cluster of small calcifications is distinguished from speckle noise by the cross-correlation, which is quite sensitive to the coincidence of a pattern of distributed small targets in both the ultrasonographic and radiographic images, notwithstanding the presence of random noise. An optical correlator is preferably used to perform the requisite high speed cross-correlations. Multiple projection vector orientations are tested for image correlation, to accomodate any probable skew between the ultrasonographic and radiographic projections, and a projection vector is found which acceptably aligns the radiographic and mammographic imagery at a fine level of allignment. After fine alignment of the mammogram and the sonogram is established in the region of interest the three-dimensional position of an individual calcification is preferably found by projecting from an identified point in the radiographic image, along the projection vector, to a voxel with extreme density in the ultrasonographic volumetric data set. In this way individual calicifications in the three-dimensional ultrasonographic image are located. An enhanced display is then generated and the information is optionally used to guide and/or verify a biopsy or other diagnostic follow up procedures.